The four dengue virus (DENV) serotypes are most important among the arthropod-borne flaviviruses in terms of human morbidity and geographic distribution. Infection with any of the DENV may lead to classic dengue fever or more severe dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS), which are increasingly common in the dengue endemic areas. DHF/DSS often occurs in patients with second, heterotypic DENV infections or in infants with maternally transferred dengue immunity. Antibody-dependent enhancement (ADE) has been proposed as an underlying pathogenic mechanism of severe dengue. ADE occurs because pre-existing sub-neutralizing antibodies and the infecting DENV form complexes that bind to Fc receptor (Fc R)-bearing cells, leading to increased virus uptake and replication. Infection with DENV or any other flavivirus induces cross-reactive, but weakly- or non-neutralizing antibodies that recognize epitopes involving the fusion peptide in the envelope glycoprotein (E). The functional activities of these cross-reactive antibodies are not well understood. Humanized monoclonal antibody (MAb) 1A5, derived from a chimpanzee Fab that had been recovered by repertoire cloning, shares properties of cross-reactive antibodies. We found that MAb 1A5 up-regulated DENV infection by a mechanism of ADE in a variety of Fc R-bearing cells in vitro. A 10- to 1000-fold increase of viral yield in human erythroleukemic K562 cells, dependent on the DENV serotype, was observed over a range of sub-neutralizing concentrations of MAb 1A5. A significant increase of DENV-4 viremia titers (up to100 fold) was also demonstrated in juvenile rhesus monkeys immunized with passively transferred dilutions of MAb 1A5. These results, together with earlier findings of ADE of DENV-2 infection by a polyclonal human serum, establish the primate model for analysis of ADE. Considering the abundance of these cross-reactive antibodies, our observations confirm that significant viral amplification could occur during DENV infections in humans with prior infection or with maternally transferred immunity, possibly leading to severe dengue. Strategies to eliminate ADE were explored by altering the antibody Fc structures responsible for binding to Fc receptors. MAb 1A5 variants, containing amino acid substitutions from the Fc region of IgG2 or IgG4 subclass reduced, but did not eliminate DENV-4-enhancing activity in K562 cells. Importantly, a 9-amino-acid deletion at the N-terminus of the CH2 domain in the Fc region that resulted from a fortuitously introduced mutation in the expression vector abrogated the enhancing activity. Among chimpanzee Fabs recovered by panning with DENV-4, Fab 5H2 is specific for DENV-4 and neutralizes the virus at a high titer in vitro. The epitope detected by the antibody was mapped by sequencing neutralization-escape DENV-4 mutants and its determinants were located in domain I on the surface of the envelope protein. Humanized full-length MAb 5H2 produced from an expression vector has been shown to be a variant containing the 9-amino-acid deletion in the Fc region. The variant MAb is particularly attractive for exploring its protective capacity in vivo. Passive transfer of antibody to monkeys was conducted to demonstrate proof of concept for protection against DENV challenge. Monkeys that received 2 mg/kg of the MAb 5H2 variant were completely protected against 100 monkey infectious dose50 (approximately equivalent to one mosquito bite) of DENV-4, as indicated by the absence of viremia and sero-conversion. These studies have important implications for antibody-mediated prevention of dengue virus infection.